This study is part of a multi-study research grant, Age and Selective Attention in Visual Search, (R37 AG02163) funded by the National Institute on Aging. The overall goal of the project is to determine the pattern of age-related change and constancy in the use of attention during the performance of visual search and classification tasks. Although the primary focus of the project is on the age-related changes that occur in the absence of significant disease, the project is also concerned with the potential role of one aspect of health status, mild essential hypertension, in mediating age-related changes in attention and cognition. All of the studies in the project involve measuring subjects' reaction time and accuracy for decisions regarding different types of visual displays. In the study conducted in the GCRC, we are comparing the performance of hypertensive and normotensive individuals in visual search performance, to test the hypothesis that the interaction and age and hypertensive status is related the memory demands of the task. In a previously conducted experiment we found that there was an interaction of age group and blood pressure group in the error rate data, such that for subjects less than 60 years of age, the error rate was higher for hypertensives than for normotensives. The reaction time data, in contrast, did not indicate substantial effects of hypertension. Thus, hypertension-related changes in cognitive performance may not be evident unless the memory demands of the task exceed some critical level. The present experiment will use a visual search task in which the number of target items held in memory (memory-set size) and the number of items in the display (display size) are varied independently. To examine the attentional demands of the task we will analyze the complete distribution of reaction time data. Previous experiments using this approach have indicated that reaction time distributions actually represent a convolution of two component distributions, one characterized by an exponential function (representing task-specific decision processes) and one characterized by a Gaussian function (representing sensory encoding and response processes). Our hypothesis is that the effects of hypertension will be most clearly evident in the exponential component of the reaction time distribution, especially when memory-set size is varied. The significance of the study is the improvement in the scientific understanding of the potential role of hypertension in age-related cognitive change, which will be important for distinguishing the effects of aging from those of disease.